A solar sail, simply put, is a spacecraft propelled by sunlight. Whereas
a conventional rocket is propelled by the thrust produced by its internal engine burn, a solar sail is pushed forward simply
by light from the Sun. This is possible because light is made up of packets of energy known as “photons,” that
act like atomic particles, but with more energy. When a beam of light is pointed at a bright mirror-like surface, its photons
reflect right back, just like a ball bouncing off a wall. In the process the photons transmit their momentum to the surface
twice – once by the initial impact, and again by reflecting back from it. Ever so slightly, propelled by a steady stream
of reflecting photons, the bright surface is pushed forward. http://www.planetary.org/solarsail/whatis.html

Solar sails are composed of large flat smooth sheets of very thin
film, supported by ultra-lightweight structures. The side of the film which faces the sun is coated with a highly reflective
material so that the resulting product is a huge mirror, typically about the size of a football field. The force generated
by the sun shining on this surface is about equal to the weight of a letter sent via first class mail. Even though this is
a very tiny force, it is perpetual, and over days, weeks, and months, this snail-paced acceleration results in the achievement
of velocities large enough to overtake and pass the Voyagers and Pioneers that are now speeding away through the outer reaches
of our solar system.

NASA has a program in place to develop
solar sail technology to a point where it can be used to implement important space exploration missions. There are a number
of missions on the NASA strategic roadmap that require this type of propellantless propulsion to achieve their objectives.
There are other classes of missions that are greatly enhanced by solar sails because these vehicles are inexpensive to construct
and can deliver such high performance propulsion.

There are important applications for solar sails beyond the science missions that NASA has planned. The National
Oceanic and Atmospheric Administration (NOAA) needs this technology to create a new class of space and earth weather monitoring
stations that can provide greater coverage of the earth and provide more advanced warning of the solar storms that sometimes
plague communications and electrical power grids. There are also a number of military missions in earth orbit that can be
enabled by low cost sailcraft.

What is solar sailing?

Solar sailing is a method of converting light energy
from the sun into a source of propulsion for spacecraft. In essence, a solar sail is a giant mirror that reflects sunlight
in order to transfer the momentum from light particles (photons) to the object one is interested in propelling. Since the
phrase "solar sails" is often confused with "solar cells", which is a technology for converting solar light into electrical
energy, we will use the term "light sail" for the purpose of this discussion.

The first suggestion that this energy could be harnessed
for propulsion came nearly 400 years ago when astronomer Johannes Kepler observed comet tails being blown by what appeared to be a solar breeze. Believing that this was evidence that
winds blew objects about in intra stellar space, he suggested that eventually ships might be able to navigate through space
using sails fashioned to catch this wind. It is now widely recognized that because space is a vacuum, winds of any significance,
do not exist. What Kepler observed was the pressure of solar photons on dust particles that are released by the comet as it
is orbiting. Photonic pressure is a very gentle force which is not observable on earth because the frictional forces in the
atmosphere are so much larger. Thus, we only expect to observe and harness the force due to the pressure of light in the vacuum
of space. ( http://solarsail.jpl.nasa.gov/introduction/index.html )